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Area forecast discussion 
National Weather Service Tallahassee Florida 
453 am EST Sat Nov 21 2009 


Synopsis... 
the 09 UTC regional surface analysis and infrared satellite imagery showed 
a 1008 mb low off the Texas coast and an area of high pressure (and 
cool dry air) over much of the southeast. A cold front trailed the 
low to the south...while a warm front extended southeastward from 
the low. A small but fairly vigorous mesoscale convective system had developed just east 
of the surface low...while isolated storms developed along the 
warm front and in the warm sector. These storms had a sheared...super 
cell appearance. Vapor imagery and upper air data showed a trough 
propagating eastward along the Texas coast...with abundant high level 
clouds streaming eastward well ahead of the approaching system. 


&& 


Short term... 
(today through monday) there have been some changes in the model 
runs compared to 24 hours ago...with a consensus shifting toward a 
weaker low with a more southerly track. In fact...the majority of 
numerical guidance all but dissipates the surface low as it quickly 
occludes near or over our forecast area Sunday night and Monday. 
While there will likely be sufficient deep layer moisture and 
synoptic scale ascent for a widespread rain event...the probability 
of a significant severe weather event appears to be diminishing. The 
latest guidance has also trended a bit slower than previous 
runs...so that the bulk of the rain will likely hold off until 
tonight...and last through Sunday morning. Some of this slow-down 
may be due to the extensive depth of dry air entrenched over our 
region...as it may take awhile for the precipitation aloft to 
moisten the very dry middle troposphere. 


Although the severe storm threat appears small at this time...it is 
not negligible. The most favorable overlap of vertical wind 
shear...deep layer moisture...lift...and instability appears to be 
between 06 UTC tonight and 18 UTC Sunday. The threat will begin 
offshore the Florida Panhandle tonight...then move east across North Florida 
and extreme south Georgia Sunday. There is little doubt that the local 
deep layer and low layer vertical wind shear profiles will be more 
than sufficient to support rotating updrafts. The question...as 
usual this time of year...is whether or not there will be enough 
instability to support surface based super cells. The consensus of 
guidance suggests the bulk of the maritime tropical airmass will 
remain just south of the coast...limiting the threat of severe 
storms over land. By the time the instability increases over land 
Sunday afternoon...the best shear and lift will likely have weakened 
and/or moved off to our northeast. The Storm Prediction Center forecasts a 2 percent 
chance of a tornado within 25 miles of a point...and a 5 percent 
chance of damaging straight line winds. We suspect there will be 
isolated super cells near and south of our coastline...but then 
they will become elevated as they move inland. It is important to 
remember that even the best models rarely resolve small scale 
details (like the leading edge of the maritime tropical air from 
the gulf) beyond 24 hours...so it remains possible that if the 
coastal front moves inland more than expected there would be an 
enhanced severe storm risk. 


Long term... 
(monday night through saturday) significant differences remain 
between the GFS and the European model (ecmwf) through the middle of the week with 
respect to the development of another low in the Gulf of 
Mexico...and the resulting potential impacts of clouds and 
possible precipitation across the tri-state area. As a 
result...overall confidence in that portion of the forecast 
remains lower. However the latest European model (ecmwf) has trended toward a 
slightly more southern development while the GFS has trended 
slightly northward. Will go with a blend as basic guidance...and 
lean towards the GFS solution for consistency...holding on to more 
clouds through Wednesday night and adjusting temperatures 
accordingly. As the second low moves east toward the end of the 
week...both models show high pressure and cooler drier air 
spreading eastward across the area through the remainder of the 
forecast period. Will show temperatures starting out above 
normal...then trending downward to near normal through the middle 
of the week...and below normal by the end of the week. 


&& 


Aviation... 
expect patches of MVFR visibilities with fog mainly across the 
Florida Big Bend and south central Georgia...and areas of low MVFR 
to IFR ceilings spreading westward into south central Georgia by 
daybreak...as reflected in the tafs for kvld and ktlh. Elsewhere 
VFR conditions will prevail with only cirrus. By afternoon a 
developing area of low pressure over the western Gulf of Mexico 
will spread thickening and lowering clouds eastward across the 
area...with MVFR ceilings and visibilities with showers developing 
across southeast Alabama and the eastern Florida Panhandle by 
later afternoon...spreading eastward across the remainder of the 
area Saturday night. 


&& 


Marine... 
with a weaker system being forecast by the models...it appears less 
likely that an advisory will be needed later tonight or Sunday. The 
00 UTC wavewatch indicates lower seas than 24 hours ago...but we 
lowered its seas forecast a foot or two more as the mean of the 
model surface winds are lower than the operational GFS...and the GFS 
ensemble mean and 21 UTC sref supports this idea of lower 
winds. 


&& 


Fire weather... 
a low pressure system developing over the western Gulf will move 
eastward today and tonight...accompanied by widespread clouds and 
showers. After this low moves east of the area on Monday...showers 
will diminish but clouds will persist..as another low pressure 
develops further south over the Gulf. Some drier air is expected 
to filter into the tri-state area by the end of the week. 
However...critical fire weather conditions are not expected to 
develop until at least next weekend. 


&& 


Preliminary point temps/pops... 
Tallahassee 72 58 71 51 71 / 20 80 50 10 10 
Panama City 70 60 72 57 70 / 30 80 40 10 10 
Dothan 65 51 64 49 67 / 30 80 50 20 10 
Albany 66 53 64 49 68 / 20 80 60 20 10 
Valdosta 71 56 69 52 72 / 10 80 70 30 10 
Cross City 76 57 76 58 77 / 10 70 60 20 10 
Apalachicola 71 62 73 57 70 / 20 70 40 10 10 


&& 


Tae watches/warnings/advisories... 
Alabama...none. 
Georgia...none. 
Florida...none. 
GM...none. 


&& 


$$ 


Long term/aviation/fire weather...Duval 
rest of discussion...Fournier 

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